Bogner Shiva blown supply protection components

[Mick Bailey]

Can anyone tell me the component values for the mains protection MOV and thermistor? Both are blown to pieces. Suspect the MOV shorted and took the thermistor with it.

This is the US voltage model.

[R.G.]

I can't tell you the thermistor value, but the MOV should be replaced with a unit rated at 150VAC, and as large a "Joule" rating as you can put in there.

MOVs have the quirk that each breakover lowers their breakover voltage a tiny amount. Eventually they drift under the peak of the AC voltage, or it drifts up over their breakover, and smoke ensues. Generally MOVS are installed after a fuse so that when breakover happens, the fuse opens and stops the smoking process. Was there a fuse before this one? If not, there should be.

The thermistor value is hard to get right. Mostly it holds down the inrush from the power switch going on. It, too, should be after a fuse. In fact, a fuse should be the first thing after the power switch. If you can tell me the power-plate rating for the amplifier I can make a reasonable guess at a thermistor. A couple of tries should get you running.

[Mick Bailey]

There is a T3A mains fuse - it didn't blow, but the HT fuse did. It goes fuse, thermistor, then the MOV is connected right across the primary. I've just run the amp up off the Variac with these components temporarily bypassed and the amp's working OK. Thankfully the PT is working.

Looking at some other schematics, it looks like Bogner are fitting a 130L20 (130v, 6500 joule) to their amps, but the AC voltage rating looks a little too close to the supply voltage for comfort. I'm happier with the suggestion of 150VAC.

Strangely (at least for Europe), there's no rating plate. I was initially thinking of a 2x6L6 Fender thermistor value of 10 Ohm/5A, but pictures of Shivas posted on the web suggest a 20mm device and so that rating would not be commensurate with a device of that size.

[Mick Bailey]

R.G., any further thoughts on the thermistor rating? I mailed Bogner but no reply.

I'm interested to know how important the coefficients are in a tube amp;

• Zero Power Resistance Rt
• Rated Zero Power Resistance R25
• Beta Value
• Temperature Coefficient of Zero Power Resistanceα
• Dissipation Coefficient δ
• Thermal time constant τ
• Max. Steady State Current
• Resistance-Temperature Characteristic
• Static V-I Characteristic

It would appear that there's a risk in the component being under specified as well as over-specified in that it could be passing to little current at operating temperature due to being to heavily rated, or running at an excessive temperature.

How important are the slope characteristics so long as the inrush to the main filter caps is controlled and the thermistor is up to operational temperature before the output tube heaters begin meaningful emission?

[Enzo]

Um, it's just a guitar amp, close is good enough.

If I had to guess at a thermistor inrush limiter, I'd just look in recent era Fender schematics for what they used. Pick one of similar power output.